The present invention relates to a method for emulsion polymerization which is characterized in that it yields a latex having a larger particle size than can typically be obtained by an emulsion polymerization process.
Methods for emulsion polymerization are well known in the art, however, these methods are generally not considered practical for producing particles larger than about one micron. The particle size is typically in the range of 0.05 to 0.3 microns.
Methods for producing larger size particles have been reported but these methods are often time consuming, require a large number of steps, and do not yield a high concentration of solids. As such they are not desirable for use in a commercial setting. For example from commonly assigned U.S. Pat. No. 4,877,761 it is known to prepare a seed emulsion and to grow larger particles by subsequently adding more monomer and surfactant to the emulsion. In one such method, three or four cycles of monomer addition may be required to grow a particle larger than one micron size. Each cycle may require several hours to perform and the product may contain less than 30% solids.
Other methods have also been tried. An emulsifier free method is disclosed by Matsumoto et al. Kobunshi Kagakir, 22, 481 (1965) and Chung-Li et al., Progress Colloid Polym. Sci., 60, 163 (1976), but this method generally yields an emulsion of low solids, small particle size, and poor stability.
Some methods have been studied for making toner particles such as microsuspension polymerization (see U.S. Pat. No. 2,932,629), but this process generally yields a broad particle size distribution in the range of 1 to 100 microns. Due to limitations on heat transfer and the large exothermic heat of polymerization, it is difficult to prepare latices having a high solids content.
One technique which has been proposed to prepare large-size latices in fewer steps is a modified version of seeded emulsion polymerization. In this process, an emulsion of seed particles is formed and the seed particles are swollen by the addition monomer. The monomer penetrates the seed particle and is polymerized there. The process is generally performed on a batch-wise basis and is not practical in producing particles having a core-shell morphology. There are also limits on the amount of monomer which can be taken up by the seed particles and on how large a particle can be formed. Ugelstad, et al., Adv. Coll. Int. Sci., 13, 101 (1980) disclose one process in which a water immiscible solvent is added to the monomer and a water miscible solvent is added to the aqueous phase to enhance the transport of monomer into the particle.
U.S. Pat. No. 4,247,434 to Vanderhoff et al. discloses another seeded emulsion polymerization process in which an inhibitor is used in the aqueous phase and an oil soluble initiator is employed to avoid new particle generation. A water soluble polymeric emulsifier and a nonpolymeric emulsifier are used to stabilize the emulsion. In this process, solid content remains low and the cycle time to produce particles larger than one micron remains large.
Another technique is dispersion polymerization. Uniform micro-sized particles in up to 40% solids content have been reported with the process using a variety of solvents as the dispersion medium with solvent soluble initiators and polymeric stabilizers.